This invention relates to a mill, and particularly but not exclusively to a cone mill for use in milling a product, the product being milled between a rotating beater and the wall of a perforated screen.
Cone mills generally comprise a conical, or truncated conical, perforated screen within which the product to be milled is located, and a rotor including at least one milling member mounted upon a rotor shaft which extends coaxially with the screen, the rotor shaft being supported by suitable means, such as a bearing housing, and being arranged to be rotated about the axis of the screen by a motor.
In order to produce the desired milled product, it is desirable to be able to adjust the spacing of each milling member from the inner wall of the screen. In known cone mills, the spacing may be adjusted by using a different rotor of different length rotor shaft, and thus either lowering or raising each milling member within the screen. Another known method is to fit one or more shims between the rotor shaft and a drive shaft via which the motor rotates the rotor shaft. Both of these methods have the disadvantage that a number of spare parts must be bought and stored, either in the form of different size rotors or shims, and that these parts may become lost, or in the case of using shims, may become loose and fall out during operation of the cone mill.
It is an object of the invention to obviate or mitigate the problems associated with such a cone mill.
According to a first aspect of the invention there is provided an adjustable length rotary drive coupling comprising a first rotary shaft provided with at least one tooth and a second rotary shaft provided with a plurality of abutment surfaces spaced apart along the length thereof associated with each tooth, one of said first and second rotary shafts being a drive shaft and the other being a driven shaft, each tooth being selectively engageable with any of its associated abutment surfaces, to enable the length of the rotary drive coupling to be adjusted.
Preferably, the second rotary shaft includes a plurality of recesses of different depths extending longitudinally of the second rotary shaft from a surface thereof facing the first rotary shaft, the base of each of the recesses defining a respective one of the abutment surfaces, with each tooth projecting towards the second rotary shaft from a surface of the first rotary shaft which faces said surface of the second rotary shaft, with each tooth being longer than at least one of its associated recesses in order to be engageable with at least one of its associated abutment surfaces.
It will be understood that, in use, the length of the rotary drive coupling is adjustable by disengaging each tooth from the selected recess or recesses, and engaging each tooth with a different one of its associated recesses. Clearly, the length of the coupling is adjustable without substituting or introducing spare parts into it.
The first shaft preferably includes two teeth which preferably extend radially of the axis of the first shaft.
The second shaft preferably includes three pairs of diametrically opposed equi-angularly spaced recesses of different depths. The recesses are preferably provided in an end face of the second shaft.
According to a second aspect of the invention there is provided a mill comprising a screen and a rotor arranged to be supported and rotated within the screen by a motor, in combination with a rotary drive coupling as defined in any of the last preceding five paragraphs, wherein the drive shaft is connected to the motor directly or indirectly and the driven shaft forms part of the rotor which also includes at least one milling member, whereby the spacing of each milling member from the screen is adjustable.
Preferably, the recesses are formed in the driven shaft and the teeth are provided on the drive shaft.
The driven shaft is preferably hollow, the drive shaft extending within the driven shaft, the drive shaft preferably including an internally screw threaded bore in its end remote from the teeth and recesses, the rotor being arranged to be secured to the drive shaft by a bolt extending through part of the rotor and engaging in the screw threaded bore.
The teeth preferably form part of a drive dog provided part way along the length of the drive shaft, the drive dog being rotatable with the drive shaft, but being prevented from performing axial movement along the drive shaft.
The screen used in conventional cone mills is normally formed from a strip of perforated material which is shaped so as to take on the form of a truncated cone, the edges of the strip being welded together, or attached to one another by an alternative known technique. The end of the screen of largest diameter is provided with an outwardly extending annular flange in order to strengthen the screen, and also to facilitate attachment of the screen to the remainder of the cone mill. The flange is generally either welded to the remainder of the screen or is formed by flaring out the end of the cone. The provision of the flange increases the cost of the screen, and often leads to hygiene problems with the cone mill.
Another known form of screen comprises a truncated cone formed of a folded and welded sheet of perforated material, the smaller diameter end of the screen being closed by an end wall welded to the screen. The screen does not include a flange at its upper end. Such a screen is supported within the cone mill by a suitably shaped cradle. In order to prevent rotation of the screen in use, a portion of a weld seam on the screen is enlarged and is arranged to sit within a recess provided in the wall of the cradle. Such an arrangement has a limited resistance to rotation in service. Also, the provision of a welded end wall at the lower end of the screen adds to the cost and complexity of manufacture of the screen.
It is an object of the third aspect of the present invention to provide a mill in which the above problems can be reduced.
According to said third aspect of the invention, there is provided a mill comprising an open bottomed screen, a rotor having at least one milling member arranged to rotate within the screen, the screen being carried by a support including an inwardly extending flange defining an aperture of diameter smaller than the upper, large diameter end of the screen, a closure member arranged to close the lower end of the screen, means for preventing the screen from rotating relative to the support, and means for clamping the upper end of the screen against the flange.
Preferably, the closure member is biassed against the lower end of the screen, the closure member preferably being supported by a plurality of arms extending from the support. The arms are preferably resilient or are resiliently mounted on the support.
The arms preferably extend onto the upper surface of the closure member, so as to engage in recesses formed in the lower end of the screen.
The clamping means is preferably arranged in such a manner as to clamp the upper end of the screen against the flange of the support, the action of clamping the screen against the flange resulting in the screen being pushed downwards against the closure member, the arms biasing the closure member against the lower end of the screen.
Whilst the present invention, in all of its above-identified aspects, is particularly applicable to cone mills, i.e. mills having a screen lying on a conical surface, it is applicable also to mills, beaters and mixers generally where there is relative movement between at least one beating, mixing or milling element and a cooperating surface, e.g. a screen or even a non-perforated surface which may be of cylindrical or any other appropriate form.